Heterojunction bipolar transistors (HBTs) are capable of ultra-high speed operation with low distortion, and are typically used for radio frequency power amplifiers (PAs) of transmitters in cell phones, local area networks (LANs), etc.
A heterojunction bipolar transistor has a thin-film multilayer structure including, for example, a sub-collector layer formed on a substrate made of semi-insulating GaAs, the sub-collector layer being made of n-type GaAs for outputting current to the outside; a collector layer formed on the sub-collector layer and made of n-type GaAs or undoped GaAs for collecting electrons; a base layer formed on the collector layer and made of p-type GaAs for controlling flow of electrons; an emitter layer formed on the base layer and made of n-type InGaP for emitting electrons; and an emitter contact layer formed on the emitter layer and made of n-type GaAs or n-type InGaAs for injecting electrons from the outside.
This thin-film multilayer structure can be produced by methods including MOVPE (metal organic vapor phase epitaxy) also known as MOCVD (Metal Organic Chemical Vapor Deposition) and molecular beam epitaxy (MBE).
Among these, metal organic vapor phase epitaxy is a method in which solid or liquid organic metal material is heated to be gasified, and supplied onto a substrate heated to a predetermined temperature, whereby the material is thermally decomposed or chemically reacted on the substrate, resulting in that a thin-film crystal is epitaxially grown on the substrate.
Molecular beam epitaxy is a method in which each of constituent elements of a thin-film crystal is vaporized in ultra-high vacuum from a different crucible, and is supplied in the form of molecular beams onto a substrate heated to a predetermined temperature, resulting in that a thin-film crystal is epitaxially grown on the substrate.
However, molecular beam epitaxy requires an ultra-high vacuum environment and is not suitable for epitaxial growth of phosphorus-based materials that require high steam pressure. Thus, for heterojunction bipolar transistors in which the emitter layer made of n-type InGaP should be epitaxially grown, only metal organic vapor phase epitaxy is used.
An epitaxial wafer for a heterojunction bipolar transistor is completed in which a thin-film multilayer structure is epitaxially grown on a substrate by means of these methods, and then, a heterojunction bipolar transistor is completed through a working process including patterning, etching, forming of electrodes (an emitter electrode, a base electrode, and a collector electrode), forming of an overcoat, and packaging.